In the Season 1 Episode 3 of The Mod Squad - Circa 1968, Pete jumps on a Sportster and the Sissy Bar looks just like the one on the Bronson Bike.

Check the circle in the bar, you can see it was a full length Schwinn bicycle bar. The episode was My What a Pretty Bus and with and original air date of 8 October 1968.

You can see shocks have been replaced with flat bars to make struts, and they bent the bottom of the sissy bar to run along the swing arm. It looks like it is held on with hose clamps and a small bracket at the fender struts. Also on the other side of the bike the exhaust pipes have been extended into what were called Hollywood pipes. They were called that because all the movies had bikes with pipes like that in the 1960's.

You cannot help but wonder if possibly this bike was used in TCB with modifications, and maybe someone like Bud Ekins thought the Sissy bar was a cool idea?